S e n s

Sensing
in Electroanalysis
Vol. 7 (K. Kalcher, R. Metelka, I. Švancara, K. Vytřas; Eds.), pp. 57−69.
© 2012 University Press Centre, Pardubice, Czech Republic.
ISBN 978-80-7395-563-2 (printed); 978-80-7395-564-9 (on-line)
Detection of Heavy Metals in Biological Samples through Anodic
Stripping Voltammetry
Vlad Buzea, Monica Florescu, and Mihaela Badea*
Transilvania University of Brasov, Faculty of Medicine, Str. Nicolae Balcescu Nr.56,
Brasov, Romania
________________________________________________________________________________
Abstract: The toxicological aspects due to the presence of heavy metals in biological samples
impose to have accurate and rapid methods for their detection. This paper is aimed to review
approaches to anodic stripping voltammetry (ASV) determination of several heavy metals (lead,
cadmium, copper, mercury, zinc) in biological matrices (blood, urine, saliva, tissue sample).
Analytical performances (LOD, data linearity range, sensitivity) of the reviewed methods were
presented for several electrochemical methods.
Keywords: Heavy metals, Lead, Cadmium, Copper, Mercury, Zinc, Electrochemical techniques
/ methods, Anodic stripping voltammetry, Samples, Blood, Urine, Saliva, Tissue.
___________________________________________________________________________
*) Author to whom correspondence should be addressed. E-mail: [email protected]
Introduction
Heavy metals can be considered as non-biodegradable pollutants of variable toxicity
depending on the type of metal, the chemical species they are constituted in, their attachment
(or lack of) to stabile substrates. Cadmium, mercury and lead for instance are foreign to all
organisms, in that they perform no biological function and are toxic for most organisms even
in trace amounts. Given their non-biodegradable nature, the toxic burden of heavy metals
tends to persist in an environment until they are eventually deposited in a chemically stable
form (as a mineral [1]).
57
During their interaction with the biological samples heavy metals show toxic potential
and this is a good reason to search for them in biological samples. Furthermore organisms
accumulate these metals storing them in organic complexes thus concentrating them along
food chains. So, they can be used as very relevant indicators of environmental contamination.
The medical and toxicological aspect is perhaps the best reason to improve detection of heavy
metals in biological samples. Because it seems that human activity is one source of heavy
metal contamination, screening and diagnosis for heavy metal poisoning is often necessary.
Anodic Stripping Voltammetry (ASV) is an electrochemical method consisting of two
fundamental steps and one intermediate period:
- the pre-concentration time - during which the analyte is electrochemically reduced and
plated on the working electrode at potentials more negative than its half-wave potential;
- the rest (or equilibrium) period needed to let the high current drop to a measurable
level (Note: Original intensity is being inappropriately high due to the faradic processes
under convective transport, which is enabled by stirring the solution or, also, by rotating
the working electrode itself);
- the analysis time is a period during which the analyte is oxidized off the electrode (the
strip) through o potential sweep past its half-wave potential and the current is displayed
as function of potential. The height of peak current is proportional to the analyte amount
stripped of the electrode.
This method's potential has already been proven as an efficient tool in the detection of
heavy metals being sensitive with lower limits of detection in the under-ppb level, having
wide data ranges and when compared to other analytical methods such as flame atomic
absorption spectrophotometry (FAAS), electrothermal atomic absorption spectrophotometry
(EAAS) or inductively coupled plasma-mass spectrometry (ICP-MS) it is simpler and less
costly [2,3,4].
In organic samples, however, ASV as well as other electrochemical methods is met
with the challenge of numerous organic compounds that can complex heavy metal ions or be
adsorbed onto the electrode surface thus fouling it and cause a decrease or total loss of signal.
Another issue is that of oligoelement trace metals such as Cu, Zn, or Co that might interfere
with foreign heavy metal detection [5,6,7]. Also solid tissue samples need to be rendered in
liquid form before commencement of electrochemical analysis [8]. This paper is aimed at
reviewing various approaches to ASV determination of heavy metals in biological matrices.
58
Heavy Metals in Blood Samples
Lead is the most frequent toxic heavy metal incriminated in human intoxication. In blood, of
the total lead amount around 95-99% is bound to red blood cells [7]. The American Center for
Disease Control provides some reference ranges for blood lead, 50μg/L for children and
100μg/L for adults, below these values no clinical consequence of lead poisoning is expected.
Though plasma and serum are simpler matrices than whole blood because they have no cells
and it may be tempting to use them in electrochemical determination as one could expect less
interference, such a determination would be irrelevant because these blood components are
much less concentrated in lead than whole blood, also there are no normal values for lead in
either serum or plasma to compare with. Since ASV can detect multiple heavy metals
including lead, a toxicological screening for heavy metals should be done in whole blood as it
allows the detection of lead which is the most common heavy metal toxicants.
Before ASV can be performed pretreatment of the sample is required. Wet digestion in
concentrated acids, if total, dissolves and oxidizes organic matter to simple inorganic
compounds sometimes this nullifies the matrix effect [8] and releases metal ions from their
bonded state making them available for reduction and then stripping at the working electrode.
However some trace heavy metals are naturally present in blood, the normal range for Cu in
human whole blood is around 1.1-1.5mg/L and double that amount in late pregnancy [9].
Copper is known to interfere with heavy metals at certain electrode setups like bismuth film
covered electrodes [6].
In 1973, Searle used an Anodic Stripping Voltammeter, (ESA) Model 2014 (graphite
mercury film plated electrode, Ag/AgCl reference Pt counter) to determine lead in capillary
blood from 200 children [10]. The sample pretreatment consisted in digestion of blood in 70%
perchloric acid for 30 minutes yielding a 1:80 dilution of blood lead in electrolyte reagent
[10]. The method was compared with AAS and the average deviation between the two
techniques was 0.9μg/L.
García-Fernández et al. [11] described a digestion method using a mixture of
nitric/perchloric/sulfuric acids, 8:8:1, and thermal treatment until the digested sample is dry
then dissolved in water (yielding a blood lead dilution of 1:50) and submitted for DPASV
(Differential Pulse Anodic Striping Voltammetry) with a anodic stripping voltammeter (VA647 workstation; Metrohm, Switzerland) electrode setup: working hanging mercury drop
electrode, reference Ag/AgCl; KCl 3M and platinum counter. This method was used for
detection of lead in blood harvested from wildlife as an indicator of environmental
contamination and has been reproduced by other authors as well [12,13].
59
Dilution of blood (anticoagulated with EDTA) then acidification with concentrated
hydrochloric acid, centrifugation and then adding reagent (HgCl2/HCl solution) to the
supernatant yielding a 1:21 dilution of blood lead (1.2M HCl and 9.2mg/L Hg2+) was
employed by Feldman et al. for square-wave anodic stripping voltammetry (SWASV) at a
disposable carbon microarray (a dielectric covered carbon layer with 287 holes exposing as
many carbon surfaces making for 287 working microelectrodes tied in parallel) with a
Ag/AgCl reference and counter electrode plated ex situ with a mercury film [14]. The data
range of the method is 3.2 to 50μg/L in blood derived solution. Results are compared with
those obtained with graphite furnace atomic absorption spectrometry (GFAAS) and the
deviations between the two ranges from -1μg/L to -7μg/L with the ASV constantly
underestimating concentration. Mechanisms of interference described are complexation of
lead and adsorption onto the electrode surface of fouling substances especially at bare carbon
and HgCl2 (from oxidation of Hg droplets on the microelectrode surface). These electrode
fouling substances are probably albumins as nitrocellulose filtration removes the effect of Hg
oxidation has on signal intensity.
Yantasee et al. [15] approached trace metal (Pb, Cd and Cu) detection in complex
matrices (blood, urine and saliva) with a metal analyzer by ASV at a Hg film coated glassy
carbon electrode under a turbulent flow of the diluted and pretreated sample. For blood the
dilution was 1:10 and the pretreatment was identical as for urine or saliva: acidification to 1M
HCl and ultrafiltration. The turbulent flow was obtained with Flow Injection Analysis (FIA)
system designed so as the jet from the flow cell inlet would hit the Hg film. The analytical
performance of the metal analyzer for blood samples was LOD (limit of detection)
of 0.46 μg/L, data range 0-20 μg/L and sensitivity of 0.033μA/ppb for lead. Concerning
interference from adsorption of blood proteins on the electrode, the jet-wall flow onto design
of the cell and ultrafiltration seem to negate the matrix effect as the use of no blood lead
spiked samples results in almost the same analytical parameters obtained on 1:10 blood
dilution (0.035 slope and 0-20 range). Concerning lead complexation by blood proteins, it is
studied on lead spiked samples that were allowed to incubate for 10 minutes. This time might
not be enough for the degree of lead binding that would occur in real exposure situations,
either chronic or acute, never the less based on the fact that Pb spiking after filtration or
before it has no influence on the concentration detected leads to authors to the conclusion that
the acidification to 1M HCl releases all lead bound to proteins.
Roda et al. [5] and Bannon et al. [16] used a common commercially available anodic
stripping voltammeter of the past two decades, the ESA model 3010 (mercury film on
graphite working electrode), to determine blood lead concentration.
60
The version A of the 3010 model was investigated by Roda et al. [5] and they found some
short comings with the apparatus accuracy as it underestimated lead in blood below 400μg/L
and overestimated it above this value also copper interference with lead is reported in this
article. The authors obtain better accuracy by using Isotope Dilution-Mass Spectroscopy
certified standards for the calibration of the anodic stripping voltammeter and by changing the
analytical signal interpretation protocol from peak area to peak height. The lead copper
interference is described as consequence of the stripping potential of the metals being very
close copper's being slightly more anodic than lead's, also the manufacturer supplied reagent
contained Cl- ions which cause the cathodic slope of the Cu peak to be shallow thus causing
peak overlap. The problem was addressed by altering the stair case striping potential
parameters implemented by the manufacturer lowering the sweep rate making for better peak
separation between Cu and Pb.
Bannon et al. [16] used a latter version the 3010B for lead determination in blood
samples (anticoagulated on EDTA) from children and compared voltammetric with GF-AAS
(Graphite Furnace Atomic Absorption Spectrometry) results and found them in accordance.
The authors used their own recipe of reagent for the measurement, blood-reagent mixture
accounted for 1:30 dilution of blood as would have been obtained for manufacturer supplied
reagent both in the case of the A and B versions of the stripping voltammeter. The recipe
employed here used chloride salts exclusively and no copper interaction is reported. Nickel
chloride was added to reagent to free lead from its EDTA complex.
So far the cited articles describe the use of mercury electrodes, which perform very
well even in the in the challenging matrix that is blood, having been extensively studied and
used in commercial devices that were proven to be on par with AAS methods [5,10,16] as far
back as almost 3 decades ago. Improvements made to these systems as far as analytic
performance brought about a reduction in the quantities of Hg used. More recent articles
reflect the tendency to abandon mercury altogether in favor of more environmentally friendly
Au, Ag, bismuth film (BiFE), polymer, carbon nanotubes (CNT) modified electrodes.
Liu et al. [6] used a Nafion coated bismuth film electrode to determine Zn, Cd and Pb
in blood samples via DPASV. The pretreatment of samples was an advanced oxidation
process (AOP) that it took about one hour and was done in a photoreactor with UV
irradiation, at the end the 0.5 mL blood sample was diluted 1:40. Then the blood reagent
mixture was submitted for DPASV; the Nafion-coated electrode was plated in situ with
300μg/L Bi3+ at -1.3V. Analytical performance was good for all metals. The highest LOD was
for Zn at 0.97 μg/L and comparison with AAS resulted in relative standard deviations of less
61
than 5% for all metals. Interference reported with copper at concentration ratio of copper-totarget analyte = 35:1, the peak for the said analyte is completely suppressed, the problem of
copper interference is assumed to be due to competition for nucleation sites between Cu and
Bi at the electrode surface and also by formation of inter-metallic compounds between Cu and
Pb. The problem is addressed by adding [Fe(CN)6]4− to the solution to complex Cu2+ ions.
Heavy Metals in Urine and Saliva Samples
Toxicology analysis for heavy metals usually requires blood samples because blood is a more
reliable indicator of exposure to heavy metals. However sample collection for both urine and
saliva is non- invasive and urine in particular reveals the plasma clearance of heavy metals,
consequently samples of these two bodily fluids may hold some usefulness in monitoring
chronically exposed individuals. In 1997, Center for Disease Control (CDC) [17] and its
recommendations concerning the lead determination in urine point out to its relevance in lead
excretion in a 8 hour period after a provocation test, namely the administration of
CaNa2EDTA (lead chelating agent). When EDTA is present in urine the incorporation of Ni2+
salts into the supporting electrolyte is recommended to free Pb2+ as Ni has a higher affinity for
EDTA than lead. For trace metal detection urine dilution has greater impact than in blood, as
urine may vary in concentration on a physiological basis so spot urine samples have almost no
relevance and it must be pooled over a time interval.
The urine matrix has been dealt with by means of acid digestion [18,19],
UV irradiation [20] and ultrafiltration [15]. Mesaroš et al. [20] proved it was possible to use a
mercury hanging drop semi-microelectrode (r = 40μm) in UV irradiated urine for
determination of lead, without supporting electrolyte.
Lead determination in urine at a BiFE glassy carbon electrode through DP-ASV was
attempted by Hop et al. [18] on 24 hours pooled urine samples from fishermen and net
weavers that held small pieces of lead in their mouths while handling nets. Blood lead was
determined by the same electro-analytical method as well and no correlation was found
between urine and blood levels. Both urine and blood samples were also analyzed with
GF-AAS and results pointed out that DP-ASV had a RSD of ±4.5% (vs. AAS).
Y. Bonfil et al. [19] used a rotating gold electrode and Subtractive Anodic Striping
voltammetry (SAVS) to determine Hg2+ in urine down to a LOD of 4nM (0.8ppb). The
authors subtracted the SWASV plot of a 0 s deposition time measurement from that of a 180 s
62
deposition time one, in order to attenuate background noise improving the quality of the
striping curve and lowering the LOD by one order of magnitude. The urine sample had been
pretreated by digestion in nitric and perchloric acids; the digestion procedure produced an
increase in LOD from 0.4nM in supporting electrolyte to 4nM in the treated urine sample this
was assumed to be due to impurities in the perchloric acid.
Khairy et al. [21] used screen printed electrodes modified with in situ Bi (1 ppm in
sample solution) to detect Cd via SWASV in saliva, the pretreatment consisted in dilution of
1:100 and acidification to pH1 the LOD was 2.3μg/L but given the dilution of 1:100 this
implies that Cd in actual samples could not be detected below 230μg/L.
Heavy Metals in Solid Tissue Samples
Before any type of electrochemical analysis can be performed on solid samples they must first
be dissolved in the supporting electrolyte, so digestion becomes a necessary step in sample
pretreatment. Digestion has the inherent disadvantages of contamination and loss of analytes
so blank reagents and, spiked samples with recovery experiments become necessary [8].
Injang et al. [22] used Bi (0.15 ppm, plating time 180s) coated CNT (5%wt) modified
SPCE
(Screen
Printed
Carbon
Electrode)
to
determine
Pb,
Cd
and
Zn
in Andrographis paniculata leaf tissue (dried leafs and capsules mad from leafs). They
experimented with two acid digestion methods namely dissolution in a mixture of
HNO3/HClO4 (optimum ratio found was 3/2) the end result was 5g of leafs (digested and
evaporated) dissolved in 5ml 1MHCl (recovery: 95%-105%); second method used HNO3 and
solid phase extraction. The SWASV measurement was done in a flow cell coupled to a system
for sequential injection analysis (SIA). The results obtained were compared with ICP-AES
measurements and found to be in accordance.
Meucci et al. [23] sought determination of Pb, Cd, Cu and Hg in Atlantic hake filets.
Lead, cadmium and copper concentrations were measured at a SPCE with the working
electrode coated by a cellulose derivative film with mercury salts, while in sample mercury
was determined at a gold ink SPE (Screen Printed Electrode). Deposition potential was -1.1V
vs. Ag/AgCl, 300s duration for the first three metals and 0.2V vs. Ag/AgCl for 200s for
mercury, stripping was done by square wave. The authors experiment with five different
recipes for fish tissue digestionworking on wet and dry muscle using 1M HNO3, 65% HNO3,
HNO3/HClO4, H2O2/HCl varying time of digestion, temperature, and purification.
63
The optimal approach (the only method that produced acceptable SWASV signals) was
digestion with H2O2 and HCl: 2 hours at 50oC for H2O2 + 2 hours for HCl at 70oC followed by
carbon column (Superlean ENVI carbon column) to remove organic matter. This method
yielded a recovery greater than 91% when used on certified reference material spiked with the
analyses of interest.
Gaston et al. [24] used a gold rotatig electrode and subtractive square wave anodic
stripping voltammetry (E deposition =0.1V vs. Ag/AgCl, deposition time =30s, E strip=1.15
vs. Ag/AgCl, step of 4mV, amplitude of 25mV and frequency of 15Hz)to detect Hg in human
hair samples. The digestion method chosen was pressurized microwave digestion carried out
in HNO3/H2O2 2:1, the supporting electrolyte was 5mM H2SO4 with 10mM NaCl and the hair
to final solution ratio was 200mg hair to 25mL solution. Various digest recipes were tested
and the end result was judged based on the spectrophotometric measurement of absorption
atλ=254nm (organic carbon absorbs at this wave length) as a degree to which the organic
matrix had been destroyed. A type of digestion mixture that gave an even lower 254nm
absorption than the above mentioned one was HNO3/HClO4/H2O2= 3:1:2, however the HClO4
was discarded as it is a very powerful oxidant and has the potential to become dangerous
when used on large quantities of organic matter. The NaCl was added to the supporting
electrolyte because the stripping peak of mercury in chloride electrolyte was proven to be 6
times higher than in the absence of chloride. The procedure was tested by determinations (by
standard addition method) carried out in certified hair samples and the regression coefficient
of the addition was R = 0.999.
Table I (given overleaf) summarizes the type of samples in which anodic stripping
voltammetry was performed for the determination of various heavy metals and the analytical
performances of the methods reviewed before.
Conclusions
Though ASV is a cost-effective, precise and accurate method especially in the detection of
heavy metals at trace amounts it is somewhat difficult to apply it to the great variety of types
of biological samples, of which we have mentioned but a few. Also it is often, if not always
necessary for biological matrices to be simplified before being submitted for analysis. Sample
preparation is often the most time consuming step in this type of determination as for the few
minutes that are spent on the actual measurement sometimes hours have to pass before the
biological sample is brought to an ASV compatible solution.
64
Table I: Analytical performance of anodic stripping voltammetry tests used for detection
of heavy metals
Analyte
Pb
Working
electrode
Electrochemical
technique
2+
2+
Cd
NCBFE
DPASV
Zn2+
Pb2+
Pb2+
Pb
2+
Hg2+
Disposable
carbon
microarray
(plated ex
situ Hg)
W-Hg film
glassy
carbon
electrode
Glassy
carbon
BiFE
Gold
rotating
electrode
SWASV
SWASV
(FIA)
Sample
LOD
(ppb)
Slope
Data
(μA/ppb)
Ref.
linearity
unless stated
range (ppb)
otherwise
Blood
AOP
pretreatment
1:40 dilution
0.13
1.7-27.9
0.148
0.09
1.3-20.5
0.182
0.97
3.5-79.6
0.562
Blood on
EDTA
1:20 dilution
1.2M HCl
5
0-50
0.03
Urine 1:2
dilution
1M HC
ultrafiltered
0.44
0-50
0.017
Blood 1:10
dilution
1M HCl
ultrafiltered
0.46
0-20
0.035
Saliva 1:2
dilution
1M HCl
ultrafiltered
1
0-10
0.463
0.7
2-100
0.15
[6]
[15]
[16]
Blood digested
in HNO3
DPASV
[18]
Urine digested
in HNO3
SW-SASV
Urine
Digested in
HNO3/HClO4
1:10 analyte
dilution
65
0.8 for
5000
rpm
0.08-80
0.059±0.005
[19]
AV-1/[Hg2+]*
Table I: ( continued )
Analyte
Cd2+
Working
electrode
W-screen
printed
graphite
(plated Bi
in situ)
Electroche
mical
method
SWASV
Sample
"artificial
saliva"
0.01M HCl
Pb2+
Cd2+
SPCNTE
(Plated Bi
in situ)
SWASV
(SIA)
5g digested then
dissolved in
5mL 1M HCl
Zn2+
Pb2+
Cd2+
Cu2+
Hg2+
Hg2+
SPCE
Modified
with Hg
film
SWASV
Screen
printed
gold
Gold
rotating
electrode
Herbs
Digested in
HNO3/HClO4
3:2
SW-SASV
Fish tissue
Digested in
H2O2 and HCl
carb column
purified
5g dry
muscle/20 mL
solution
Human hair
Digested in
HNO3 and
H2O2
LOD
(ppb)
4.3
Slope
Data
(μA/ppb)
linearity
Ref.
unless stated
range (ppb) otherwise
20-90
0.2
2-100
0.8
2-100
11
12-100
0.054
[21]
0.388
(2-18 ppb)
0.668
(20-100)
0.586
(2-18ppb)
0.799
(20-100)
0.175
(2-18ppb)
0.243
(20-100)
[22]
0.3
0.31
1
0.10
0.5
1-1000
0.9
0.384
0.25
[23]
0.04
6.7-46.7
1
[24]
0.2g in 25mL
solution
Legend: *analytical signal was not the peak height but a sum of the absolute values of slopes at the
two inflexion points at the peak base. Then, instead of A / concentration, we have a formula** when
adding the electrode area as 0.00661cm2, the deposition time 180s, and the velocity 5000 rpm.
**)
66
Also, certain methods of preparation, such as mineralization by the acid digestion, have
significant tendencies to cause the sample contamination and the corresponding controlling
steps must be implemented in order to reveal the unwanted contamination and/or analyte loss
in the event that it occurs.
Finally, while electrochemical techniques have great potential for miniaturization,
portability and a possibility to be employed in on-site analysis, the cited cases of the sample
preparation still require a laboratory.
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